A laboratory animal nutritional research program is being conducted with various strains and stocks of small animals maintained in conventional and specific pathogen-free environments. The primary objectives of the program are to develop and evaluate open formula rations (open formula-complete list of ingredients) that can be used to improve the nutritional status of animal colonies and to identify differences in nutrient requirements among various strains and stocks of animals. A seris of factorial-designed feeding trials are conducted to determine the effect of varying concentrations of major and minor nutrients on growth, reproduction, and maintenance of different rat, mouse, guinea pig, and rabbit strains.